Reflecting surface for incandescent lamps



April 1953 P. H. MITCHELL 2,636,143

REFLECTING SURFACE FOR INCANDESCENT LAMPS Filed May 21, 1949 I rn en for Pewc [/a/ H. Hifc/re it Patented Apr. 21, 1953 REFLECTING SURFACE FOR INCANDESCENT LAMPS Percival H. Mitchell, Toronto, Ontario, Canada Application May 21, 1949, Serial No. 94,595

e Claims. (01. 313-113) mventionrelates to improvements; in reflectors: for use with incandescent lamps, and particularly to annular reflectors arranged concentrically with flat-ring incandescent filaments an optical axis, for reflecting and. concentrating'. light rays along the axis.

The principal object of the invention is to provide a reflector form which will systematically obtain from a luminous flat ring filament a relatively higher concentration of reflected light on "the illuminated field thanfis obtainable with conventional elliptical and parabolic reflectors of -the 'same size. a

The principal feature of the inventionis the employment of a reflecting surface concentric with a fiat ring type incandescent filament, which surfacew-fll have a geometrical relationship with respect to points on or adjacent to the circumfi'erence of the'fll'ament to reflect and direct from all'pointson its surface light rays emanating 7 i'rom such filament points to be incident on the illuminated field normal to the optical axis of the reflector surface and remote therefrom at a given point or small area of desired light concentration while at the same time reflecting and directing'light rays from substantial lengths of the i'llament adjacent said filament points to be incident in circular zones immediately surrounding the remote" concentration point or area.

More specifically in the case of an ellipsoidal reflector surface-the invention provides an ellipsoidal reflecting surface which will, from all points on its surface, reflect and direct light rays emanating from a point on the circumference of a luminous flat ring'fllament to be'incident on the normal illuminated field at the remote elli'ptical focus, at the same time reflecting "and directing light rays from adjacent substantial "lengths of the luminous filament to be incident in circular zones immediately surrounding the l remote focus. 'l'his invention is applicable to mirrored incan descent lamps and to reflectors of metal, mirrored "glass or other material, exterior to an incanf descent lamp bulb. With reierence to the accompanying drawings, Figure 1 is a cross sectional representation of a reflector and filament of mirrored incandescent lamp according to this invention.

Figure 2 is a diagram of" light distribution from the lamp shown i'nFigure'l. Annular reflectors of parabolic or elliptical form, in combination with flat ring filaments, either in mirrored lamps or exterior'to incandescent lamps, are in common use. Flat ring filaat the remote focus. reflecting points, will be directed to the illumiments are an accepted form for general service lamps for l20-volt circuits and in commercial sizes up to 200 watts. Reflectors in combination with such filaments have each reflecting point reflecting and directing a luminous image of the filament, in its specific aspect, to be incident on the illuminated field and the resulting illumination is an overlapping of these individual images. When reflecting points are in the plane of the filament the reflected images are straight lines;

when the reflecting points are on either side of the filament plane the reflected images are of flattened oval form.

- Flat ring filaments are not complete circles but may be four-fifths of a, circle, the open space heinghetween the supporting conducting Wires leading to the filament. The filament, in plan, may

not-be circular but may be four-fifths of apentagon. For purposes of description and design, in the following, the filament is considered as a complete circle and the eifect of divergence-from this shape is shown later. For any oval aspect of the filament as viewed from a point on the reflector the luminous intensity of any short length of filament is considered as directly proportionate to the whole circumferential length. This is ac 'ceptable as a general rule.

The conventional optical systems employing elliptical and parabolic reflectors with flat ring filaments consists of thereflector andthe fiat filament arranged concentrically on an optical'axis with a focus on the axis in the plane of :the'fllament and a remote focus on the axis at a finite distance from the focus at the filament for elliptical reflectors, and at infinity for parabolic reflectors.

Straight line filament images reflected from points on the reflector lying in the plane of the filament will be incident on the illuminated field with a midway point on the straight line image Oval images, from all other nated field to be surrounding the remote focus which, coincides with the intersection of major and minor axes of the oval images. Luminous oval images from conventional reflectors then do not contribute light at the remote focus, the point where it is desired to have a high light concentratlon.

' my reflector system each of the innumerable luminous oval images contributes light at th remote focus to give the desired high co'n centration of light at this point.

Referring to the drawings. illustrating the manher in which the desired light concentration is ure 1, on'these lines is at D. luminous image from point in Figure 1. --oval marked it is the filament image from P, the

achieved, Figure 1 shows the elements comprising the optical system for a mirrored lamp of practical general dimensions and constructed in accordance with my invention. MNOP and M'N'O'P indicate opposite sides of the mirrored. surface of a lamp bulb. A and B are two points on the flat circular filament as cut by the plane of the figure and the filament is concentric with the optical axis the plane of the filament, YY, crossing XX at C, O and 0' represent the zone of the reflector lying in the plane of the filament. The diameter of the reflector in the zone through P and P is chosen to suit the lamp specifications. D is a remote focus on the optical axis XX spaced from C. Z2 is an illuminated plane at D. normal to XX. The curvature of MNOP is elliptical with an axis through A and D and foci A and D. The line MNOP rotated about the aXis XX describes the ellipsoidal surface of the re'fiector, then B and D are the foci for the elliptical The fiat circular filament A B appears as a straight line as Viewed from point 0. The filament appears as a flattened oval from points M, N and P and with the geometrical relationship described between the filament and the reflecting surface a point on the flat circular filament at A is a point on a luminous oval image at D as reflected and directed from reflecting points M, N or P and the respective oval image will contact D when incident on the illuminated plane ZZ.

A luminous image from 0 will appear at D as a straight line through D.

If the foci had been C and D as in conventional elliptical reflectors the point at A when reflected from O, on a straight line image, would be at D but the oval images would have the intersection of their major and minor axes, corresponding to C, directed to D and the luminous filament image would be spaced away from D.

In Figure 1 the continuation of the lamp bulb outward from P to P may be constructed as desired and the lamp neck beyond M and M and various lamp details may also fit any particular requirements of the lamp.

Figure 2 is a diagram showing filament images on the illuminated plane, ZZ in Figure 1, at the remote focus D as reflected and directed from points on the reflector at M, N, O and P in Figure 1, and from three other corresponding series of points on the reflector spaced 90 apart. These figures are angularly to scale, that is, the aspect of the filament from the respective points has the length of the straight line. image from O and the lengths of the major and minor axes'of the ovals,

from other points, and the ovals themselves, ex-

pressed as the degrees which their axes subtend with reference to the respective reflecting'points M, N, O and P; D represents the remote focus on the illuminated plane. Straight lines representthe straight line images from points corresponding to O.

(For clarity these are shown as paired lines instead of superimposed lines); the position of A, B and corresponding points in Fig- The line 0 is the The ovals m and n from points M and N in Figure 1 are, naturally, reflected to be on the opposite side of the straight line image from the oval P. The opposite paired straight line, and ovals, are from points 0 and M, N and P in Figure 1, while the other paired figures are from opposite points on the reflector ninety degrees from the plane of Figure 1. These straight line and oval images all contact the point D. Intermediate points on the .tinuously smooth reflector;

4 reflector will reflect and direct images to the illuminated plane and again these will contact the point D.

The oval figures m, n and 73 each contain a central point marked me, no and pc corresponding to the position in the respective oval of the point C in Figure 1 had it been a luminous source. If C in Figure 1 had been the immediate focus with D as the remote focus'as in conventional reflectors these points would be directed to be coincident with D and the actual luminous oval circumference would be spaced away from D and would not contribute to the illumination at D as in my construction.

F,.G and H are circles with D as centre and are drawn with radii equivalent of one, two and three degrees respectively to indicate the respecrespective circle a length of filament image from the opposite side of the oval will also be contained'.

There are" several alternatives to .the use of A and D as foci in Figure 1.

Alternatively to the arrangement in Figure 1, the point B on the filament can be used as one focus and the axis for the elliptical arc corresponding to MNOP would be BD. This will reverse the positions of the ovals with respect to the corresponding straight line images in Figure 2.

A further alternative is to have the arc OP with foci at A and D and the arc MNO with foci atB and D. As the respective immediate foci A and B lie in the same plane with the point 0 the curvatures of the two arcs join to be continuously smooth. The oval images on the illuminated plane will all lie on the same side with respect to the straight line image. Similarly the arc OP can have foci at B and D and the arc MNO can have ,foci at A and D. If the shift from A to B, or vice versa, as the immediate focus is made at some other point than 0 there will be definite-change in curvatures and the join will not produce a conby introducing. a short circular are between the two ellipticalarcs the reflector can be made continuously smooth. .1

Instead of using A or B in Figure 1 as the immediate focusywith D asthe remote focus, for

"the development'of the elliptical arc MNOP-,-- 1a focus" adjacent to A or B may be used. Assume .a focus in the plane of-A B spaced one eighth the distance from A to B. The straight line images in Figure 2 will pass through point Dbut the equivalent position of A on the oval images will be spaced away from D a distance equal to approximately'one eighth ofthe minor diameter of the oval. The concentration-will-not be as great as when A is used as focus so that-all the oval images pass through D but will'be greater than when C is used'as focus as all the oval images will fallwithin a -much more -confined 1 area with the focusadjacent A instead of C-, which confined area I term a substantially point area to distinguish from the much larger area'occupied by the oval images with focus C. Figure 1 shows a mirrored glass bulb type of reflector. The same opticalarrangement maybe asses-rs ustd with grenades exterior to: an incandescent lam in which case the. diameter of reflector relative to diameter of the ring filament may be increased When the nominal reflector diameter is two or three times the diameter of reflector in Figure 1, and with the same filament diameter, thefilament images will be reduced to the order: of one half or one third the dimensions shown in Figure 2 and thcconcentration within limiting zones will be increased totwice or three times the concentrations obtainable with the smaller reflector.

Filament images are shown, diagrammatically in Figure 2 as lines. As the filament itself, genma as a coiled wire, may have a thickness of one fiftieth of an inch the actual thickness of filament image line for Figure 2 will then be with the scale used of the order of re inch for images from point M in Figure l, decreasing to 1 6 inch for images from point P. These line image thicknesses are for points onv the image corresponding to A, and for points on the images corresponding to B the thicknesses will be less so that any oval truly represented by lines, of thickness to scale, will have varying thickness throughout its circumference. I

The figures show the specific conditions for an elliptical reflector with a remote focus at six feet. The remote focus may be close to infinity' and then the ellipse is virtually a parabola.

Within the scope of practical design of reflectors for use with flat ring filaments if the remote focus is at 50 feet distance the reflector maybe designed as a parabola although it actually forms an ellipse having an eccentricity approaching 1.,

An illuminated zone equivalent to one degree radius at 50 feet has a diameter at the zone" of approximately 21 inches which is in excess of the reflector diameters contemplated for thesjlde' vices. If the reflector in Figure 1 is designed as a parabola as noted above the parabolic axis is through A or B as focus and is parallel with the optical axis XX. The parabolic equivalent of the arc MNOP rotated about XX describes the reflector surface. Point A on the respective filament images is then rotated about XX extended at the illuminated plane on a circle corresponding to the circular zone of the reflector in which the reflecting point lies. When the reflector diameter is of the order of, say, one per cent of the distance to the illuminated plane the displacement o'fl thi s point on the filament images, away from the axis, is of small consequence.

As stated, filaments of the type used in. combination with these reflectors are not complete circles due, to the gap between the conducting wires. The filament images are then notcomplete ovals but will appear with about one fifth of their circumferences missing. The .-.lamp lumens are then obtained from these shorter lengths and diagrams of filament images would be properly shown as four fifths of the complete ovals with the missing sector migrating systematically through the illuminated field. Noffr natter what arrangement of foci are used there=will be some lack of uniformity of illuminationfover the field. The distribution of these non-luminous sectors will be less localized when the foci A'and D or B and D are used, as in Figure l, as oval images from opposite points of the reflector arepaired to be on either side of the remote focus and thus have two separate orbits of migration, scattering the trace of the missing sector over a larger area.

When the filament is in the form of four sides a Pen a e of. ei rfivfi :9 circle'theovals of Figure 2 would appear diagram matically as four joined chords approximating four fifths of each oval. There would not be systematic contact of these figures with the remote focus but in most applicationsthe effect would be acceptable. What Iclaim as my invention is:

1. Light concentrating means comprising in combination a fiat ring-like luminous -filament,

and an annular reflecting surface concentric with said filament, said reflecting surface having a geometric relationship to said filament and the axis thereof such that every point on said surface lies on the curve ofan ellipse having an immediate focus lying on the circumference of said filament and a remote focus lying on or immediately adjacent to the axis of said filament and said reflecting surface, the two said foci and the said axis being in the same plane and a line joining the two said foci forming an acute angle with the said axis subtended by a radius of said filament whereby all points on said reflecting surface reflect and direct light rays from their immediate foci on said filament to be incident on a normal illuminated field within a substantially point area at said remote focus on said axis, and reflect and direct light rays from substantial lengths from said filament adjacent their said immediate foci to be incident on the illuminated field in zones immediately surrounding said point area.

2. In an incandescent lamp, an evacuated bulb coated to provide an annular reflector surface, a flat ring-like luminous filament centered on the axis of said surface, said bulb being shaped so that substantially every point on its reflecting surface lies on an ellipse having an immediate focus located adjacent the circumference of said filament and a remote focus outside said bulb and on the axis of said surface and filament, the two said foci and the said axis being in the same plane and a line joining the two said foci forming an acute angle with the said axis subtended by a radius of said filament whereby all points on said reflecting surface reflect and direct light rays from points on the circumference of said filament adjacent their immediate foci to be incident on a normal illuminated field Within a substantially point area at said remote focus, and reflect and direct light rays from substantial lengths of said filament adjacent their said immediate foci to be incident on the illuminated field in zone imme diately surrounding said point area.

3. In combination with an annular reflector and a flat ring-like luminous filament centered on the axis of said reflector surface, said reflector surface being ellipsoidal having a locus of immediate foci coinciding substantially with the circumference of said filament and a remote focus lying on said reflector axis with lines joining said immediate foci and said remote focus defining a cone centered on said axis and subtended by said filament whereby each point on said reflector surfilament a reflector surface having a configura- -tion corresponding to a surface generated by the axis.

revolution about the. axis of saidfilament of a continuous curve substantially all sections of which form an ellipse having an" immediate focus different immediate foci.

s; A device. as claimed'in claim 4 in which said continuous curve is formed of sections having immediate foci on opposite sides'of said filament PERCIVAL H. MITCHELL.

a ier nce @ited in w file of this patent UNITED STATES PATENTS Number Number Name 'Date '9 Ballman July 17,1928 Claus May 5,1931

Gilleland et a1. Dec. 8, 1931 Rivier Nov. 20, 1934 Cook, Jr. Mar; 8,1938 Birdseye Jan. 17', 1939 Singer Apr. 23, 1946 FOREIGN PATENTS I 1 Country Date Great Britain May 14, 1931 Germany June 27, 1935 

